WASHINGTON — South Korean and U.S. researchers said Wednesday they had cloned a human embryo and extracted from it sought-after cells called embryonic stem cells. The cloning was not intended to make human babies, but the first step toward developing cures for diabetes, Parkinson's and other diseases, the researchers said.

The experiment, the first published report of cloned human stem cells, means so-called therapeutic cloning is no longer a theory but a reality. Supporters of medical cloning say it can transform medicine, offering tailored and highly effective treatments for diseases. They say it could eventually lead to grow-your-own organ transplants.

The stem cells taken from the tiny embryos, known as blastocysts, have the potential to develop into any kind of cell or tissue in the body.

“Our approach opens the door for the use of these specially developed cells in transplantation medicine,” Woo Suk Hwang of Seoul National University in Korea, who led the study, said in a statement. Funding for the experiments came from a private donor, whom Hwang did not name.

'A very impressive study'Outside experts on cloning praised the work. “It is a very impressive study. It obviously represents a major medical milestone,” said Dr. Robert Lanza, who has helped lead cloning experiments at Massachusetts company Advanced Cell Technology. “I think it could help spur a medical revolution.”

Working with Hwang was Dr. Jose Cibelli, formerly of Advanced Cell Technology and now a researcher at Michigan State University. Hwang has conducted extensive research in the field of cloning and is currently working to preserve the Korean tiger as well as to develop a miniature pig and cattle resistant to mad cow disease.

Wednesday's report is sure to revive international controversy over whether to ban all human cloning. Critics say it involves destroying a human embryo, however tiny, and is thus unethical. The administration of President Bush and supporters in Congress are seeking to outlaw the technology both in the United States and worldwide.

Scientists have cloned sheep, cattle, mice and other species but have had trouble cloning a human being. Last year Advanced Cell Technology said it had created a human cloned embryo but it had not grown enough to become a source of stem cells.

The company is still trying but has not reported publicly on its progress.

Embryonic stem cells are the body’s building blocks, cells from which all other tissue types spring. They’re present in an embryo only days after conception and are ethically sensitive because culling stem cells destroys the embryo.

Writing in the journal Science, Hwang and colleagues said they created the clone using eggs and cumulus cells donated by Korean women. Cumulus cells are found in the ovaries and in some species have been found to work especially well in cloning experiments.

The researchers said they also tried to use cells taken from men, specifically the ear lobes, but did not have any success. So, for now, this cloning process can be used only with women.

Powerful master cellsStem cells are found throughout the body and are a kind of master cell. But adult stem cells are difficult to find and to work with.

Seoul National Univ. via Reuters

This microscopic photo shows an injection of a somatic cell into a nuclear removed-human egg cell during an experiment in Seoul.

Many scientists believe blastocysts -- stem cells taken from days-old embryos -- have much greater potential. Each one, when grown correctly, can be directed to become any kind of cell or tissue at all.

The researchers used a process called nuclear transfer, which involves removing the nucleus from an egg cell and replacing it with the nucleus of a so-called adult cell -- in this case a cumulus cell.

The scientists say they succeeded largely because of using extremely fresh eggs donated by South Korean volunteers and gentler handling of the genetic material inside them.

At a press conference Thursday at the annual conference of the American Association for the Advancement of Science, held this year in Seattle, Hwang said he did not think it would be easy for other researchers to duplicate the technique. "In my humble opinion, it's not so easy to imitate our technology because the human oocyte (or egg) is very sticky."

In the experiment, Hwang and his colleagues cloned each woman using her own egg cell and her own cumulus cell, so the clones were 100 percent copies of each woman.

They activated the egg cells using a chemical process, which started the eggs growing as if they had been fertilized by a sperm and got 30 embryos to grow to the blastocyst stage.

At this stage, approximately 100 cells, the stem cells should be removable.

The history of cloningThey pulled stem cells from one of the blastocysts and managed to get them to grow into a variety of different cells including eye cells, muscle cells, bone and cartilage.

It’s elegant work that provides long-anticipated proof that human therapeutic cloning is possible, said stem-cell researcher Dr. Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass.

Years of additional research are required before embryonic stem cell transplants could be considered in people, he stressed.

Ban sought on reproductive cloningStill it’s sure to renew debate over whether all forms of human cloning should be banned. The House last year voted to do that, but the Senate stalled over whether there should be an exception for some research.

U.S. scientists almost universally want a ban on cloning for reproduction, because the high rate of birth defects in cloned animals shows the technique is still too dangerous.

During Thursday's press conference, Hwang urged all countries to implement such a ban and said the technique should not be used to create human babies.

South Korea recently passed laws governing cloning research and Hwang said he has stopped his experiments while he waits for a license from the government that would allow him to continue.

Some scientists said the publishing of the report would make it easier for maverick cloning researchers, notably Kentucky fertility expert Dr. Panos Zavos, to create a human baby.

“(Zavos has) got the cookbook now. It’s scary,” Lanza said.

But others argued the technique itself is not new, only the achievement of a successful outcome is.

"It is a recipe only in the sense that to catch a turtle is the recipe for turtle soup," said Donald Kennedy, editor of the journal Science.

MSNBC's Alan Boyle, the Associated Press and Reuters contributed to this report